Blue/violet semiconductor laser devices which are fabricated by using III-V group nitride-type semiconductor materials such as gallium nitride are key devices for realizing ultra-high density recording with optical disk apparatuses, and are about to reach a practical level. Increases in the output power of blue/violet semiconductor laser devices not only make possible a fast write to optical disks, but also open doors to new fields of technology, such as application to laser displays.
Conventionally, GaN-type semiconductor laser devices employing sapphire substrates have been developed. In recent years, however, use of nitride semiconductor substrates, e.g., GaN substrates, for fabricating GaN-type semiconductor laser devices has been studied. For example, Non-Patent Document 1 discloses a GaN-type semiconductor laser device which is at a practical level. Hereinafter, with reference to FIG. 20, a process for producing the conventional GaN-type semiconductor laser device will be described.
As shown in FIG. 20, this semiconductor laser device is fabricated by using a GaN substrate 1601 whose principal face is covered with an SiO2 mask layer 1602. A plurality of stripe openings 1603 are formed in the SiO2 mask layer 1602. On the GaN substrate 1601, a GaN layer 1604 is grown through selective lateral growth (ELO) using metal-organic vapor phase epitaxy technique (MOVPE technique). The GaN layer 1604 epitaxial grows on the principal face of the GaN substrate 1601 that is exposed through the individual stripe openings 1603 of the SiO2 mask layer 1602. However, the GaN layer 1604 grows not only in the direction perpendicular to the principal face of the substrate, but also in horizontal/lateral directions, until coming in contact with one another to form one layer.
On the GaN layer 1604, by MOVPE technique, an n-GaN crystal 1605, an n-AlGaN cladding layer 1606, an n-GaN optical guide layer 1607, a multi-quantum well (MQW) active layer 1608 composed of Ga1−xInzN/Ga1−yInyN (0<y<x<1), a p-GaN optical guide layer 1609, a p-AlGaN cladding layer 1610, and a pGaN contact layer 1611 are stacked. The GaN layer 1604, which has been formed through an ELO step, includes edge dislocations at portions where the respective stripes of GaN having grown along the lateral direction cohere with one another, and the edge dislocations also extend from the GaN layer 1604 into the semiconductor layers that are grown thereon.
On the p-GaN contact layer 1611, a ridge stripe having a width of about 1.5 to 10 μm is formed in a region where the edge dislocations do not exist. Thereafter, the ridge stripe is buried with an SiO2 layer 1613 on both sides.
Thereafter, a p electrode 1612 composed of e.g. Ni/Au is formed on the ridge stripe and the SiO2 layer 1613. Note that a part of the aforementioned multilayer composite is etched until the n-GaN crystal 1605 is exposed, and an n electrode 1614 composed of e.g. Ti/Al is formed on the surface of the n-GaN crystal 1605 that has been exposed through this etching.
In the semiconductor laser device shown in FIG. 20, when the n electrode 1614 is grounded and a voltage is applied to the p electrode 1612, holes are injected from the p electrode 1612, and electrons are injected from the n electrode 1614, toward the MQW active layer 1608. As a result, a population inversion of carriers occurs within the MQW active layer 1608, thus creating an optical gain and causing laser oscillation in the 400 nm wavelength band. The oscillation wavelength varies depending on the composition and film thickness of the thin Ga1−xInxN/Ga1−yInyN film which is the material of the MQW active layer 1608.
In the semiconductor laser device disclosed in Non-Patent Document 1, the width and height of the ridge stripe are adjusted so that laser oscillation occurs in the fundamental transverse mode along the horizontal direction. In other words, by introducing a difference in the optical confinement factor between the fundamental transverse mode and the high-order modes (i.e., modes which are higher than the first order), oscillation in the fundamental transverse mode is enabled.
The GaN substrate is fabricated as follows, for example.
First, by MOCVD technique, a single-layer film of GaN is grown on a sapphire substrate. Thereafter, by a method such as hydride VPE (H-VPE), a thick film of GaN is deposited on the GaN single-layer film. After the GaN film is grown to a sufficient thickness, the sapphire substrate is peeled to obtain a GaN substrate.
A GaN substrate which has been fabricated by the above method has a problem in that it includes dislocations, e.g., edge dislocations, screw dislocations, and mixed dislocations, at a density of about 5×107 cm−2. When dislocations are present at such a large density, it is difficult to obtain a highly-reliable semiconductor laser device.
The GaN layer 1604 shown in FIG. 20 is grown in order to obtain a GaN layer having few dislocations. By adding the ELO step of the GaN layer 1604, the dislocation density in the GaN crystal can be reduced to about 7×105 cm−2. By forming an active region (current injection region) above a region with few dislocations that has been thus formed, it becomes possible to improve the reliability of the semiconductor laser device.
Note that, the p electrode 1612 and the n electrode 1614 are both formed on the same surface of the GaN substrate 1601 in the conventional nitride semiconductor laser device shown in FIG. 20. In such a laser device, an injected current which is necessary for laser oscillation does not need to flow through the GaN substrate 1601. Therefore, the GaN layer 1604 does not need to be electrically conductive, nor is the GaN 1604 subjected to any intentional impurity doping.
Patent Document 2 discloses, after forming stripe ridges on a principal face of a substrate such as SiC, selectively growing a GaN crystal on the upper face of each ridge.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-9004
[Patent Document 2] Japanese National Publication No. 2002-518826
[Non-Patent Document 1] Japan Journal of Applied Physics (Jpn. J. Appl. Phys.), vol. 39, p. L648 (2000)